There is a need to measure the purity of fluids in many different circumstances. In addition to the familiar examples of monitoring the quality of air and water, chemicals used for industrial processing and laboratory or analytical purposes must meet certain standards of purity. It is particularly important in processes for treating fluids, such as for processing raw water into potable water, or for processing wastewater so that it is safe for release into the environment, to measure purity both before and after the fluid is treated. That is, measuring purity in fluid before it is treated is often desirable to determine how to treat the fluid, and measuring purity at the end of treatment is often necessary as a quality control, or to confirm conformance to regulatory standards.
Devices used for measuring fluid purity in general, and for identifying and quantifying the amount of impurities in particular, commonly use light as a probing mechanism. Such devices are generally referred to as photometers. A specific type of photometer is the spectrophotometer, which permits adjustment of the light frequency (i.e., wavelength), for making measurements at multiple frequencies. The term “spectrophotometer” as used herein includes any photometer, including reflectometers, transmissometers, and nephelometers, adapted for this purpose.
Light that is used to irradiate material may either be reflected by the material, transmitted through the material, or absorbed by the material. Where the light is absorbed by the material, the material may also emit light in response, or fluoresce. In devices used to measure purity, one of three basic measurement methodologies following from these potential interactions of the light with the matter is generally employed. These methodologies measure the parameters absorption, reflectance, and fluorescence and are referred to herein as absorption, reflectance, and fluorescence methodologies. According to the various methodologies, a light detector is disposed with respect to a light transmitter so that the detector is optimally positioned to be responsive to the associated parameter.
For example, for responding to absorption, the detector is typically disposed directly opposite the transmitter, to detect light that is undeflected from its original path; for responding to reflectance, the detector is typically disposed directly adjacent or next to the transmitter, to detect light that is reflected from surfaces; and for responding to fluorescence, the detector is typically disposed at an angle from the transmitter, to detect omnidirectional fluorescent emissions.
However, as can be readily appreciated, in each of the above detector/transmitter configurations, the detector will in general respond to at least one other parameter. In the absorption methodology, the detector response will be affected by both reflectance and fluorescence as well as absorption; in the reflectance methodology, the detector response will be influenced by fluorescence as well as reflectance; and in the fluorescence methodology, the detector will be influenced by reflectance as well as fluorescence.
Accordingly, it is typical in analytical laboratories to pre-process the sample being tested, or to adjust the measurement methodology, to minimize or eliminate responses due to parameters that are not being measured. For example, in the absorption and fluorescence methodologies, the sample can be clarified to eliminate particulates that would introduce reflectance, and in the reflectance and absorption methodologies, the light can be filtered at both the transmitter and the receiver to limit the response to frequencies in which fluorescence is expected.
Testing fluid quality in a laboratory as a control mechanism has serious drawbacks, as explained in the present inventor's U.S. Pat. No. 5,304,492, incorporated by reference herein in its entirety. To solve these problems, the '492 patent discloses an in-situ spectrophotometer having a single transmitter/detector configuration that is indicated as being capable of use for measuring absorption, reflectance, and fluorescence. The device provided for measurement of any the three desired parameters in essentially real-time, in the flow stream of the fluid being tested. The device remains extremely advantageous for measuring a selected one of these different parameters. However, as recognized and explained herein, there is a need for a light returning target for a photometer for measuring more than one of these parameters in the same device.